This invention relates to steam turbine bucket technology and, more specifically, to a radial loading spring used in the installation of steam turbine reaction type buckets in steam turbine rotor grooving.
Current practice for radial loading of steam turbine reaction style buckets involves inserting each bucket into a retaining groove in the steam turbine rotor, inserting a loading pin in a tightly controlled radial gap between the bottom of the bucket and the rotor groove, and then hammering the pin such that the pin plasticly deforms in the rotor radial direction and loads the bucket radially against a hook in the retaining groove. For each bucket, there is a loading pin and each loading pin must be hammered manually until the bucket does not move in the rotor groove. This hammering operation, however, introduces an opportunity to damage the bucket as well as the rotor. Accordingly, there is a need for an improved radial loading technique that provides parts reduction, rotor assembly time reduction, and consistent radial loading of the buckets against the rotor groove hook without danger of damage to the buckets and/or rotor.
This invention replaces the loading pin technique with radial loading spring segments that eliminate the hammering operation and reduce the number of discrete parts required for bucket installation. In the exemplary embodiment, the new radial loading spring segment may have a xe2x80x9cCxe2x80x9d cross-section, but the final spring cross-section could vary in order to achieve the desired loading force on the buckets. The span or arcuate length of the spring segments could be as much as 360xc2x0, which would mean that only one spring segment per annular spring groove would be required. More than one spring groove (for example, a pair of side-by-side annular grooves) could be utilized in order to achieve a higher loading force on the bucket, and more than one spring segment may be utilized to fill the one or more 360xc2x0 spring grooves in each turbine stage. One advantage of utilizing shorter spring segments is ease of installation of the spring segment in the groove, and ease of installation of the buckets in the groove.
In the preferred arrangement, numerous radial slices (also referred to as slots) are made in each spring segment, thus effectively forming multiple individual springs in each segment, so that the compression of the spring under one particular bucket is localized under that bucket, and not affect the spring loading on adjacent buckets. The radial slices can be made perpendicular to the segment centerline, or at the same angle as the bucket dovetail rhombus angle.
Accordingly, in one aspect, the invention relates to a loading spring segment for radially loading a turbine bucket within a turbine rotor groove, the loading spring comprising a substantially circular metal sheet with a gap between opposed edges of the sheet, the sheet defining an arcuate segment in a length direction of the spring segment; and a plurality of radial slots in the sheet, spaced along the length direction to thereby create a plurality of individual springs in the arcuate segment.
In another aspect, the invention relates to a turbine rotor and bucket assembly comprising a rotor formed with a bucket retaining groove about a periphery thereof; a plurality of buckets, each having a mounting portion including a radially inner face received within the bucket retaining groove; an annular spring groove located in a base portion of the bucket retaining groove, and at least one radial loading spring segment seated in the annular spring groove, radially interposed between the base portion of the bucket retaining groove and the radially inner face portion of at least one of the plurality of buckets; the radial loading spring element comprising a metal sheet of substantially circular cross-section, with a gap between opposed edges thereof, and at least one radial slot in the circular sheet to thereby form at least two discrete springs within the spring segment.
In still another aspect, the invention relates to a method of assembling a turbine bucket to a rotor wherein the turbine bucket is formed with a male dovetail and the rotor is formed with a peripheral female dovetail groove, wherein the female dovetail groove has a base portion formed with an annular spring retaining groove, the method comprising a) locating a radial loading spring segment of predetermined arcuate length in the spring retaining groove; b) twisting the bucket to enable the male dovetail to pass into the female dovetail; c) applying a radial force to the bucket to thereby compress the radial loading spring segment; and d) twisting the turbine bucket to a desired orientation where the male dovetail is fully seated within the female dovetail.